Image capturing apparatus and method of controlling the same

ABSTRACT

An image capturing apparatus comprises an image pickup device with a plurality of pixels each of which includes at least two photoelectric conversion portions, a readout unit configured to read out a first image signal and an added signal obtained by adding the first image signal and a second image signal, a subtraction unit configured to subtract the first image signal from the added signal, a focus detection unit configured to detect a focus state based on the first and the second image signal, and a limiter unit configured to suppress an output of the first photoelectric conversion portion and an output of the second photoelectric conversion portion not to exceed a predetermined threshold, wherein the limiter unit suppresses the output of the first photoelectric conversion portion and the second photoelectric conversion portion for different color filters.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image capturing apparatus having anauto-focusing function.

2. Description of the Related Art

There is conventionally known a technique of performing focus detectionat high speed and accuracy by using a focus detection method adopting aphase difference detection method in an image capturing apparatusincluding an imaging optical system and an image pickup device. In afocus detection technique using the phase difference detection method, apupil division unit divides a light beam radiated from the imagingoptical system into at least two areas, and the light beam of each areais photoelectrically converted to obtain a pair of focus detectionsignal strings of two images. A focus shift amount in a predeterminedfocal plane, that is, a defocus amount is detected from the relativeimage shift amount between the two signal strings. In focus detectionusing the phase difference detection method, in an in-focus state, thestrengths of the signal strings of the two images coincide with eachother, and the relative image shift amount should also be zero. However,due to vignetting of the focus detection light beam caused by theimaging optical system and various aberrations of the imaging opticalsystem, the coincidence of the two images deteriorates, resulting in afocus detection error. Due to vignetting and various aberrations, theproportional relationship between the defocus amount and the relativeimage shift amount between the two images deteriorates. To perform focusdetection at high accuracy, therefore, it is necessary to eliminate theerrors. A technique for this purpose has also been proposed.

On the other hand, there has been proposed a technique in which atwo-dimensional CMOS sensor or the like is used as an image pickupdevice to arrange focus detection pixels for phase difference detectionon the sensor. An image capturing pixel and a focus detection pixel arearranged on the same plane. Therefore, when the image capturing pixel isin the in-focus state, the focus detection pixel is also in the in-focusstate. For this reason, in principle, no relative image shift occursbetween two images for phase difference detection in the in-focus state,and thus an error hardly occurs. Since, however, the focus detectionpixel includes two photoelectrical conversion portions, a circuit forreading out accumulated pixel signals is complicated. A method ofsuppressing complication of a circuit for reading out pixel signals isdescribed in Japanese Patent Laid-Open No. 2008-103885.

In the technique described in Japanese Patent Laid-Open No. 2008-103885,each of a plurality of focus detection pixels each including twophotoelectric conversion portions outputs a signal obtained by addingoutput signals from the two photoelectric conversion portions.

According to Japanese Patent Laid-Open No. 2008-103885, however, when avalue outputtable from the focus detection pixel is exceeded(saturated), crosstalk occurs due to leakage of charges between the twophotoelectric conversion portions of the focus detection pixel.Crosstalk causes an output signal to include a signal in addition to asignal obtained by photoelectrically converting the light beam from theimaging optical system, thereby disabling correct focus detection.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the aboveproblems, and suppresses the possibility that focus detection isdisabled even if an image capturing apparatus including an image pickupdevice capable of detecting a phase difference has a saturated pixel.

According to the first aspect of the present invention, there isprovided an image capturing apparatus comprising: an image pickup devicewith a plurality of pixels each of which includes at least twophotoelectric conversion portions including a first photoelectricconversion portion for receiving light having passed through a portionof an pupil area of an imaging optical system and a second photoelectricconversion portion for receiving light having passed through a differentportion of the pupil area of the imaging optical system, and a colorfilter of a predetermined color; a readout unit configured to read out,from the image pickup device, a first image signal obtained from thephotoelectric conversion portion, and an added signal obtained by addingthe first image signal obtained from the photoelectric conversionportion and a second image signal obtained from the second photoelectricconversion portion; a subtraction unit configured to obtain the secondimage signal by subtracting the first image signal from the addedsignal; a focus detection unit configured to detect a focus state of theimaging optical system based on the first image signal and the secondimage signal; and a limiter unit configured to suppress an output of thefirst photoelectric conversion portion and an output of the secondphotoelectric conversion portion not to exceed a predeterminedthreshold, wherein the limiter unit suppresses the output of the firstphotoelectric conversion portion and the output of the secondphotoelectric conversion portion for different color filters not toexceed the predetermined threshold with respect to the first imagesignal and the second image signal.

According to the second aspect of the present invention, there isprovided a method of controlling an image capturing apparatus includingan image pickup device with a plurality of pixels each of which includesat least two photoelectric conversion portions including a firstphotoelectric conversion portion for receiving light having passedthrough a portion of an pupil area of an imaging optical system and asecond photoelectric conversion portion for receiving light havingpassed through a different portion of the pupil area of the imagingoptical system, and a color filter of a predetermined color, the methodcomprising: a readout step of reading out, from the image pickup device,a first image signal obtained from the photoelectric conversion portion,and an added signal obtained by adding the first image signal obtainedfrom the photoelectric conversion portion and a second image signalobtained from the second photoelectric conversion portion; a subtractionstep of obtaining the second image signal by subtracting the first imagesignal from the added signal; a focus detection step of detecting afocus state of the imaging optical system based on the first imagesignal and the second image signal; and a limiter step of suppressing anoutput of the first photoelectric conversion portion and an output ofthe second photoelectric conversion portion not to exceed apredetermined threshold, wherein in the limiter step, the output of thefirst photoelectric conversion portion and the output of the secondphotoelectric conversion portion for different color filters aresuppressed not to exceed the predetermined threshold with respect to thefirst image signal and the second image signal.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the arrangement of an image capturingapparatus according to an embodiment of the present invention;

FIG. 2 is a view showing an array of pixels of an image pickup deviceaccording to the embodiment;

FIG. 3 is a circuit diagram showing the image pickup device according tothe embodiment;

FIG. 4 is a view showing the optical principle of an imaging opticalsystem according to the embodiment;

FIGS. 5A to 5D are graphs each for explaining the relationship betweenan incident light amount and an output signal;

FIGS. 6A to 6D are graphs each for explaining the relationship between apixel and an output signal according to the embodiment; and

FIG. 7 is a flowchart illustrating the operation of the image capturingapparatus according to the embodiment.

DESCRIPTION OF THE EMBODIMENTS

An embodiment of the present invention will be described in detail belowwith reference to the accompanying drawings.

FIG. 1 is a block diagram showing the arrangement of an image capturingapparatus according to the embodiment of the present invention. FIG. 1shows an electronic camera which integrates an imaging optical systemand a camera body with an image pickup device, and can record a movingimage and a still image. Referring to FIG. 1, reference numeral 101denotes a first lens group which is disposed at the front end of theimaging optical system for forming an object image and held to bemovable along an optical axis; 102, a stop which adjusts the diameter ofits opening, thereby adjusting the amount of light in imaging and alsofunctions as an exposure time adjustment shutter in imaging of a stillimage; and 103, a second lens group. The stop 102 and the second lensgroup 103 are driven together along the optical axis, and, ininterlocking with the movement operation of the first lens group 101,provide a variable magnification effect (a zoom function). Referencenumeral 105 denotes a third lens group which carries out focusadjustment by moving along the optical axis; and 106, an opticallow-pass filter which is an optical element for reducing false color andmoire of a captured image.

Reference numeral 107 denotes an image pickup device which includespixels capable of performing focus detection and is composed of a CMOSsensor and its peripheral circuits. A two-dimensional, single-CCD colorsensor in which light-receiving pixels arranged M pixels in thehorizontal direction by N pixels in the vertical direction and anon-chip Bayer arrangement primary color mosaic filter is disposed isused as the image pickup device 107. Each pixel of the image pickupdevice 107 includes a plurality of photoelectric conversion portions anda color filter.

Reference numeral 111 denotes a zoom actuator which carries out avariable magnification operation by rotating a cam cylinder (not shown)manually or by the actuator to drive the first lens group 101 throughthe third lens group 105 along the optical axis; 112, a stop actuatorwhich controls the diameter of the opening of the stops 102 and adjuststhe amount of light for imaging, and also controls the exposure time inimaging of a still image; and 114, a focus actuator which drives thethird lens group 105 along the optical axis to adjust the focus.

Reference numeral 121 denotes a CPU which includes an arithmetic unit,ROM, RAM, A/D converter, D/A converter, and communication interfacecircuit for performing various kinds of control of the camera body. Inaddition, based on predetermined programs stored in the ROM, the CPU 121drives the various circuits of the camera, and executes a series ofoperations of focus control (AF), imaging, image processing, recording,and the like.

Reference numeral 122 denotes an image pickup device drive circuit whichcontrols the image capturing operation of the image pickup device 107and A/D-converts obtained image signals to transmit the converted imagesignals to the CPU 121; 123, an image processing circuit which performsprocessing such as color interpolation, γ conversion, and imagecompression on the images obtained by the image pickup device 107; and124, a phase difference calculation processing circuit serving as afocus detection unit, which obtains the image shift amount between an Aimage and a B image by correlation calculation using, as an AF A imagesignal and an AF B image signal, signals obtained from the twophotoelectric conversion portions of each pixel of the image pickupdevice 107, thereby calculating a focus shift amount (focus state).Furthermore, reference numeral 125 denotes a focus drive circuit whichcontrols to drive the focus actuator 114 based on the focus detectionresult to drive the third lens group 105 along the optical axis, therebyperforming focus adjustment; 126, a stop drive circuit which controls todrive of the stop actuator 112, thereby controlling the opening of thestop 102; and 127, a zoom drive circuit which drives the zoom actuator111 according to the zoom operation of the user.

Reference numeral 128 denotes a limiter unit which suppresses the addedsignal of the A image signal and the B image signal not to exceed apredetermined threshold; and 129, a control unit which generates an Aimage and (A+B) image by controlling the limiter unit when the addedsignal of the A image signal and the B image signal exceeds thepredetermined threshold in the limiter unit 128.

Reference numeral 131 denotes a display unit such as an LCD whichdisplays information about the imaging mode of the camera, a previewimage in imaging, a confirmation image after imaging, and an in-focusstate display image in focus detection; 132, an operation unit which isconstituted by a power switch, imaging start switch, zoom operationswitch, imaging mode selection switch, and the like; and 133, adetachable flash memory which records captured images including a movingimage and a still image.

FIG. 2 is a view showing an array of the pixels of the image pickupdevice 107 according to the embodiment of the present invention. FIG. 2shows a state when observing, from the imaging optical system side, arange of six rows in the vertical (Y) direction and eight columns in thehorizontal (X) direction of a two-dimensional CMOS area sensor. TheBayer arrangement is applied to color filters. Color filters of greenand red are alternately provided for pixels on an odd-numbered row fromleft. Furthermore, color filters of blue and green are alternatelyprovided for pixels on an even-numbered row from the left. A circle 211i represents an on-chip microlens. Each of a plurality of rectanglesarranged within the on-chip microlenses represents a photoelectricconversion portion which is divided into a first photoelectricconversion portion 211 a for receiving light having passed through aportion of the pupil area of the imaging optical system and a secondphotoelectric conversion portion 211 b for receiving light having passedthrough another portion of the pupil area of the imaging optical system.

In this embodiment, the photoelectric conversion portion of each of allthe pixels is divided into two areas in the X direction. With respect toa photoelectric conversion signal of each divided area, a signal can beindependently read out from the first photoelectric conversion portion211 a for each color filter but a signal cannot be independently readout from the second photoelectric conversion portion 211 b. The signalof the second photoelectric conversion portion 211 b is calculated bysubtracting the signal of the first photoelectric conversion portion 211a from a signal read out after adding the outputs of the firstphotoelectric conversion portion and the second photoelectric conversionportion.

The signals from the first photoelectric conversion portion 211 a andthe second photoelectric conversion portion 211 b can be used not onlyfor focus detection using the phase difference detection method in amethod (to be described later) but also for generating a 3D(3-dimensional) image formed by a plurality of images having parallaxinformation. On the other hand, information obtained by adding theoutputs of the divided photoelectric conversion portions is used as ageneral captured image.

Pixel signals when performing focus detection using the phase differencedetection method will now be described. In this embodiment, themicrolens 211 i and the divided photoelectric conversion portions 211 aand 211 b of FIG. 2 pupil-divide a light beam radiated from the imagingoptical system. Assume that an image composed by concatenating outputsfrom the photoelectric conversion portions 211 a in a plurality ofpixels 211 within a predetermined range arranged on one row is set as anA image which is the first image, and an image composed by concatenatingoutputs from the photoelectric conversion portions 211 b in the samepixels is set as a B image which is the second image. In this case,detecting, by correlation calculation, the relative image shift amountbetween the A image as the first image and the B image as the secondimage which have been generated in this manner can detect a focus shiftamount in a predetermined area, that is, a defocus amount.

FIG. 3 is a view showing the arrangement of a readout circuit in theimage pickup device 107 according to this embodiment. Reference numeral151 denotes a horizontal scanning circuit; and 153, a vertical scanningcircuit. Horizontal scanning lines 152 a and 152 b and vertical scanninglines 154 a and 154 b are arranged in the boundary portions between therespective pixels. Signals are read out from the respectivephotoelectric conversion portions to the outside via these scanninglines.

Note that the image pickup device 107 according to this embodiment hastwo types of readout modes. The first readout mode is called anall-pixel readout mode, which is used to capture a high-resolution stillimage. In this case, signals are read out from all the pixels. Thesecond readout mode is called a thinning readout mode, which is used toonly record a moving image or display a preview image. In this case,since the number of pixels required is smaller than the total number ofpixels, the apparatus reads out signals from only pixels remaining afterthinning out the pixel groups at a predetermined ratio in both the X andY directions.

It is only necessary to read out an (A+B) image for a general image forimaging. To detect a phase difference in a plurality of distancemeasurement areas, however, an A image signal and a B image signal areread out and the image shift amount between the A image and the B imageis detected by correlation calculation, thereby calculating a defocusamount.

FIG. 4 is a view for explaining the conjugate relationship between theexit pupil plane of the imaging optical system and the photoelectricconversion portions of the image pickup device 107 which are arrangednear a portion corresponding to an image height of 0, that is, thecenter of an image plane. The photoelectric conversion portions 211 aand 211 b in the image pickup device and the exit pupil plane of theimaging optical system are designed with on-chip microlenses to have aconjugate relationship. In general, the exit pupil of the imagingoptical system almost coincides with a plane on which an iris stop forlight amount adjustment is placed. On the other hand, the imagingoptical system according to this embodiment is a zoom lens having avariable magnification function. If a variable magnification operationis performed, an imaging optical system of some optical type changes insize or distance from the image plane of the exit pupil. FIG. 4 shows astate in which the focal length of the imaging optical systemcorresponds to a middle position between the wide-angle end and thetelephoto end, that is, “Middle”. An exit pupil distance in this stateis represented by Zmid. Assuming that this distance is a standard exitpupil distance Znorm, the shape of the on-chip microlens is designed.

Referring to FIG. 4, reference numeral 101 denotes a first lens group;101 b, a lens barrel member which holds the first lens group; 105, athird lens group; 105 b, a lens barrel member which holds the third lensgroup; 102, the stop; 102 a, an aperture plate which defines an openingdiameter in a full-aperture state; and 102 b, stop blades for adjustingthe opening diameter in a stopped-down-aperture state. Note that themembers 101 b, 102 a, 102 b, and 105 b which act to limit the light beampassing through the imaging optical system are illustrated as opticalvirtual images when observed from the image plane. In addition, acomposite opening near the stop 102 is defined as the exit pupil of thelens, and the distance from the image plane is defined as Zmid, asdescribed above.

The pixel 211 includes, from the lowermost layer, photoelectricconversion portions 211 a and 211 b, wiring layers 211 e to 211 g, acolor filter 211 h, and the on-chip microlens 211 i. The on-chipmicrolens 211 i projects the photoelectric conversion portions 211 a and211 b onto the exit pupil plane of the imaging optical system.Projection images are represented by EP1 a and EP1 b.

If the stop 102 is in the full-aperture state (for example, F2.8), theoutermost portion of the light beam passing through the imaging opticalsystem is represented by L(F2.8). The projection images EP1 a and EP1 bare not eclipsed by the stop opening. On the other hand, if the stop 102is in the stopped-down-aperture state (for example, F5.6), the outermostportion of the light beam passing through the imaging optical system isrepresented by L(F5.6). The outer sides of the projection images EP1 aand EP1 b are eclipsed by the stop opening. Note that at the center ofthe image plane, the eclipsed states of the projection images EP1 a andEP1 b are symmetrical with respect to the optical axis, and the amountsof light received by the photoelectric conversion portions 211 a and 211b are equal.

A measure taken when the output value of the photoelectric conversionportion of this embodiment exceeds an upper limit value (is saturated)will be explained next. Each of the photoelectric conversion portions ofeach pixel receives a light amount from the light beam passing throughthe imaging optical system, and outputs a signal corresponding to thelight amount by photoelectric conversion. In the case of ahigh-luminance object with a large amount of light, however, the upperlimit value of the light amount which can be accumulated in thephotoelectric conversion portions 211 a and 211 b is exceeded to causeleakage of charges to the adjacent photoelectric conversion portions,resulting in crosstalk. Crosstalk occurs between the A image signalgenerated from the photoelectric conversion portion 211 a and the Bimage signal generated from the photoelectric conversion portion 211 b,resulting in image shift amount error between the A image signal and theB image signal. Therefore, an error occurs in the defocus amountobtained by detecting the image shift amount by correlation calculation,thereby disabling setting of a desired object in the in-focus state.

In this embodiment, in a process of generating a B image signal, a Bimage signal is generated by subtracting an A image signal from an (A+B)image signal. An outputtable upper limit value is set for an imagesignal. In this embodiment, the same upper limit value is set for theimage signals of the A, B, and (A+B) images. When the image signal ofthe A image reaches the outputtable upper limit value, the output signalof the (A+B) image also reaches the upper limit value. As a result, the(A+B) image signal and the A image signal also reach the upper limitvalue. That is, when the A image signal takes the upper limit value, the(A+B) image signal takes the same upper limit value, and the B imagesignal is generated by subtracting the A image signal from the (A+B)image signal, thereby outputting 0. In this case, the A image signaltakes the upper limit value and the B image signal takes 0, resulting ingeneration of an error image signal. Therefore, even if the image shiftamount between the A image and the B image is detected by correlationcalculation to calculate a defocus amount, a desired object cannot beset in the in-focus state. Furthermore, even if the A image signal hasnot reached the upper limit value, when the (A+B) image is in asaturated state, the image signal is lost upon generating a B image. Inthis case, even if a defocus amount is calculated from the image shiftamount between the A image and the B image by correlation calculation, adesired object cannot be set in the in-focus state.

As described above, to set a high-luminance object in the in-focus stateeven if each pixel is saturated, it is necessary to control the imagesignals so that the A image signal and the (A+B) image signal do notreach the upper limit value. In the embodiment, the limiter unit 128 forsuppressing the A image signal and the B image signal not to exceed thepredetermined threshold is provided and the control unit 129 forcontrolling the limiter unit 128 is provided, thereby controlling theimage signals not to reach the upper limit value.

In this embodiment, since the A image signal is converted into aluminance signal by adding the pixel values of the color filters ofgreen (to be referred to as G1 hereinafter) and red (to be referred toas R hereinafter) of the odd-numbered rows and those of the colorfilters of blue (to be referred to as B hereinafter) and green (to bereferred to as G2 hereinafter) of the even-numbered rows, a threshold isset for each of G1, R, B, and G2. With this arrangement, even if thevalue of the specific color G1, R, B, or G2 reaches the upper limitvalue, the limiter unit 128 sets a threshold, and the control unit 129suppresses each image signal not to exceed the threshold.

For the B image signal, the limiter unit 128 sets a threshold withrespect to its luminance signal. This is done for the following reason.That is, processing of generating a B image for each of G1, R, B, and G2is equivalent to processing of temporarily storing each of the A imageand (A+B) image for each of G1, R, B, and G2, and generating G1, R, B,and G2 of a B image. The scales of a circuit for storing a signal, acircuit for generating a signal, and the like become large. Therefore,the signal of a B image is generated from the luminance signals of the Aimage and (A+B) image. For this reason, for the B image, the limiterunit 128 sets a threshold with respect to its luminance signal, and thecontrol unit 129 suppresses the luminance signal not to exceed thethreshold.

A saturation determination method for controlling a signal from eachphotoelectric conversion portion of the embodiment not to exceed theupper limit value will be described with reference to FIGS. 5A to 7.Each of FIGS. 5A to 5D shows an incident light amount from the imagingoptical system and the output signal of the image pickup device. Theabscissa represents the incident light amount and the ordinaterepresents the output signal. A solid line indicates the (A+B) image, adotted line indicates the A image, and a one-dot dashed line indicatesthe B image. Each of FIGS. 6A to 6D shows an example of an actual signalin the in-focus state. The abscissa represents pixels on an arbitraryrow and the ordinate represents an output signal. FIG. 7 is a flowchartillustrating an operation according to the embodiment.

FIG. 5A shows a case in which saturation determination is not performedand FIG. 5B shows a case in which saturation determination is performed.In the case of the actual signal, FIG. 6A shows a case in whichsaturation determination is not performed and FIG. 6B shows a case inwhich saturation determination is performed. In the interval from 0 toA1 in which the incident light amount is small in FIG. 5A, since eachpixel signal does not reach the upper limit value even if the incidentlight amount is photoelectrically converted (an area up to A1), both theA image signal and the B image signal reflecting the incident lightamount can be output. In the interval after A1 in which the incidentlight amount is large in FIG. 5A, when the (A+B) image signal exceedsthe upper limit value, the signal of the B image itself decreasesbecause the B image is generated by subtracting the A image from the(A+B) image. That is, in this embodiment, the upper limit values of therespective image signals are equal. Therefore, when the A image signalexceeds ½ the upper limit value, the B image signal starts to decreasedue to the influence of the A image. That is, when the A image signalexceeds ½ the upper limit value, the B image signal decreases due to theinfluence of the A image signal. In other words, as the A image signalincreases, the B image signal should also increase. However, after the Aimage signal exceeds the upper limit value, the B image signal decreasesas the A image signal increases. That is, the B image signal unwantedlychanges inversely. Consequently, as shown in FIG. 6A, the imagecoincidence between the A image and the B image extremely deteriorates.As described above, even if correlation calculation is performed, theimage coincidence deteriorates and thus the calculated defocus amount isnot correct, thereby disabling setting of the desired object in thein-focus state.

A case in which saturation determination is performed will be describedwith reference to FIG. 5B. FIG. 5B assumes that the image signals of theA image and B image take the same value. The limiter unit 128 sets, as athreshold, a value which sets the output of the image signal of the Aimage to ½ the upper limit value, and the control unit 129 suppressesthe image signals not to exceed the threshold. With this arrangement,both the image signals of the A image and the B image are equal to orsmaller than ½ the upper limit value, and thus the image signal of the(A+B) image does not reach the upper limit value. As described above,performing saturation determination enables correction calculationwithout impairing the image coincidence between the A image and the Bimage in the in-focus state, as shown in FIG. 6B, thereby calculating acorrect defocus amount.

In an area in which the peripheral portion of the image pickup device,that is, the image height is large, the diameter of the exit pupilbecomes small due to eclipse (vignetting) of the imaging optical system.The received light amount of the pixel decreases, and the received lightamounts of the two photoelectric conversion portions become unequal. Asthe opening diameter of the stop decreases, the nonuniformity of thereceived light amount becomes significant. As described above, thereceived light amounts of the two photoelectric conversion portions 211a and 211 b in one pixel may be different from each other. Saturationdetermination when the values of the A image signal and the B imagesignal as the signals from the two photoelectric conversion portions 211a and 211 b are different from each other will be explained withreference to FIGS. 5C, 5D, 6C, and 6D.

FIG. 5C shows a case in which saturation determination is performed fornot the B image but the A image and FIG. 5D shows a case in whichsaturation determination is performed for both the A image and the Bimage. Each of FIGS. 5C and 5D shows a case in which the B image signalis larger than the A image signal. Even if the A image signal is equalto or smaller than ½ the upper limit value, the B image signal may havealready exceeded ½ the upper limit value, and thus the signal of the(A+B) image may reach the upper limit value. Since the B image signal isgenerated by subtracting the A image signal from the (A+B) image signal,a false signal is unwantedly output to the B image signal because the(A+B) image signal has exceeded the upper limit value. Consequently, asshown in FIG. 6C, the image coincidence between the A image and the Bimage extremely deteriorates, and thus it is impossible to calculate acorrect defocus amount from the image shift amount by correlationcalculation.

In this embodiment, as shown in FIG. 5D, a threshold is also set for theB image signal. More specifically, the threshold is set so that the Bimage signal is equal to or smaller than ½ the upper limit value. Bysetting the threshold for the B image signal, a situation in which onlythe B image signal exceeds ½ the upper limit value is avoided. This cangenerate the B image signal while preventing a situation in which one ofthe A image signal and the B image signal exceeds ½ the upper limitvalue to cause a false signal due to saturation of the (A+B) imagesignal to mix into the B image signal. Consequently, as shown in FIG.6D, it is possible to calculate a defocus amount from the image shiftamount between the A image and the B image by correlation calculationwithout impairing the image coincidence between the A image and the Bimage, thereby enabling a high-luminance object, for which a pixel issaturated, to enter the in-focus state.

An operation according to this embodiment will be described withreference to the flowchart shown in FIG. 7. The process starts in stepS101. In step S102, a signal from the image pickup device 107 is readout, and the process advances to step S103. In step S103, R, G1, B, andG2 signals of the A image signal are generated, and the process advancesto step S104. In step S105, it is determined for each of the output R,G1, B, and G2 signals of the A image whether the signal output exceeds athreshold. If the threshold is not exceeded, the process advances tostep S107; otherwise, the process advances to step S106. In step S106,the A image signal exceeding the threshold is set to a predeterminedvalue equal to or smaller than the threshold, and saturationdetermination is equally performed for the R, G1, B, and G2 signals ofthe A image on the next line. Upon completion of saturationdetermination for all the A image signals or the A image signals withina predetermined AF frame, saturation determination is terminated in stepS107, and the process advances to step S108. In step S108, all the R,G1, B, and G2 signals of the A image are added to generate a luminancesignal, and the process advances to step S109. In step S109, theluminance signal of the (A+B) image is generated by adding the luminancesignal of the A image and that of the B image, and the process advancesto step S110. In step S110, to start saturation determination for the Bimage signal, the luminance signal of the B image is generated bysubtracting the luminance signal of the A image from that of the (A+B)image. In step S112, it is determined whether the luminance signal ofthe B image exceeds a threshold. If the threshold is not exceeded, theprocess advances to step S114; otherwise, the process advances to stepS113. In step S113, the luminance signal of the B image is set to apredetermined value equal to or smaller than the threshold, andsaturation determination is performed for the B image luminance signalon the next line. Upon completion of saturation determination for allthe B image signals or the B image signals within the AF frame, theprocess advances to step S114 and ends in step S115.

As described above, after the (A+B) image signal is converted into aluminance signal, the B image is generated and saturation determinationis performed for the luminance signal of the B image. Although it ispossible to perform saturation determination for the R, G1, B, and G2signals of the B image, an additional memory for storing these signalsis required, thereby increasing the circuit scale. As in thisembodiment, by adding the R, G1, B, and G2 signals of each of the (A+B)image and the B image to obtain a corresponding luminance signal, andperforming saturation determination for the B image using its luminancesignal, it is possible to decrease the circuit scale and calculate adesired defocus amount. The above procedure allows calculation of adefocus amount from the image shift amount between the A image and the Bimage by correlation calculation.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2013-082499, filed Apr. 10, 2013, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image capturing apparatus comprising: an imagepickup device with a plurality of pixels each of which includes at leasttwo photoelectric conversion portions including a first photoelectricconversion portion for receiving light having passed through a portionof an pupil area of an imaging optical system and a second photoelectricconversion portion for receiving light having passed through a differentportion of the pupil area of the imaging optical system, and a colorfilter of a predetermined color; a readout unit configured to read out,from said image pickup device, a first image signal obtained from saidphotoelectric conversion portion, and an added signal obtained by addingthe first image signal obtained from said photoelectric conversionportion and a second image signal obtained from said secondphotoelectric conversion portion; a subtraction unit configured toobtain the second image signal by subtracting the first image signalfrom the added signal; a focus detection unit configured to detect afocus state of the imaging optical system based on the first imagesignal and the second image signal; and a limiter unit configured tosuppress an output of said first photoelectric conversion portion and anoutput of said second photoelectric conversion portion not to exceed apredetermined threshold, wherein said limiter unit suppresses the outputof said first photoelectric conversion portion and the output of saidsecond photoelectric conversion portion for different color filters notto exceed the predetermined threshold with respect to the first imagesignal and the second image signal.
 2. The apparatus according to claim1, wherein said limiter unit suppresses the output of said firstphotoelectric conversion portion for at least one of color filters ofred, green, and blue not to exceed the predetermined threshold withrespect to the first image signal.
 3. The apparatus according to claim1, wherein said limiter unit suppresses a signal obtained by addingsignals of said second photoelectric conversion portions correspondingto color filters of red, green, and blue so as not to exceed thepredetermined threshold with respect to the second image.
 4. A method ofcontrolling an image capturing apparatus including an image pickupdevice with a plurality of pixels each of which includes at least twophotoelectric conversion portions including a first photoelectricconversion portion for receiving light having passed through a portionof an pupil area of an imaging optical system and a second photoelectricconversion portion for receiving light having passed through a differentportion of the pupil area of the imaging optical system, and a colorfilter of a predetermined color, the method comprising: a readout stepof reading out, from the image pickup device, a first image signalobtained from the photoelectric conversion portion, and an added signalobtained by adding the first image signal obtained from thephotoelectric conversion portion and a second image signal obtained fromthe second photoelectric conversion portion; a subtraction step ofobtaining the second image signal by subtracting the first image signalfrom the added signal; a focus detection step of detecting a focus stateof the imaging optical system based on the first image signal and thesecond image signal; and a limiter step of suppressing an output of thefirst photoelectric conversion portion and an output of the secondphotoelectric conversion portion not to exceed a predeterminedthreshold, wherein in the limiter step, the output of the firstphotoelectric conversion portion and the output of the secondphotoelectric conversion portion for different color filters aresuppressed not to exceed the predetermined threshold with respect to thefirst image signal and the second image signal.